The 220 kV Siji Line (Sihui-Zhaoqing Duanzhou) is the main line connecting the Zhaoqing power grid with the provincial grid. It was put into operation in July 1995, with a total length of 39.612 km and a total of 113 towers, including 35 straight poles, 53 straight iron towers, and 25 tension towers. The terrain along the line is characterized by 79.8% marshland and 20.2% mountainous terrain, with most towers located in marshland areas. [b]1 Problems with the Tower Foundation[/b] The No. 012 tower of the 220 kV Siji Line of the Zhaoqing Power Bureau is of type KGU1-45, with a nominal height of 45 m and a total height of 67.5 m. The foundation design consists of four plain concrete slabs (each with a base size of 5.6 m × 5.6 m) for support, and the soil foundation was not reinforced. Because the foundation is divided into four independent sections, it experiences varying degrees of displacement and settlement due to the influence of various combined forces and changes in surrounding loads. In March 2001, on-site measurements showed that the displacement of the tower top along the line reached 725 mm, and bending deformation of the tower materials was observed in many places. Significant skewness was also observed in the suspension insulator strings on the tower wings. [b]2 Cause Analysis[/b] 2.1 Geological Impact On-site investigation revealed that the 220 kV four-terminal line tower 012 was located between a fishpond along the line and a river embankment along the opposite direction, with a relatively high water level. On May 19, 2000, a borehole of 24.85 m was drilled near the foundation of tower 012, reaching the bearing layer at -17 m below ground level. However, the soil had poor cohesion, consisting of weathered residual siltstone with moderate weathering. 2.2 Uneven Settlement of Foundation Since the foundation of Tower 012 consists of four independent large slabs, and the bottom depth of each slab is -2.0 m, with a 6.9 m thick peat soil layer at the bottom, the ample water immersion in the nearby fishpond, and the fluid plasticity and wet expansion-dry shrinkage characteristics of the bearing soil layer, the two slabs closest to the fishpond tilt towards the fishpond side under the combined action of long-term positive pressure and changing loads. [b]3 Problem Solving[/b] 3.1 Solution Selection 3.1.1 Relocation A new site is selected near the original foundation, and a more stable bored pile foundation is constructed to build a new tower of the same model or to move the original tower to the new foundation. Reconstructing the foundation using bored piles is a feasible method, and from a design perspective, the possibility of foundation displacement or settlement is very low. Its disadvantages are: a) High cost. Because relocating the site would inevitably require occupying land in nearby fishponds, resulting in substantial land acquisition compensation, and given the limited power outage time and the lack of on-site tower hoisting capabilities, a new tower of the same model must be erected. Preliminary estimates suggest the total project cost exceeds 1 million yuan. b) Power outages are required. During tower erection and dismantling, a relatively long power outage is necessary, lasting approximately 48-72 hours, significantly impacting social and economic benefits. 3.1.2 Reinforcement and Correction on the Original Foundation: After on-site investigation and research, relevant engineering technicians proposed a method of reinforcement using bored piles. This involves reinforcing the original four plain concrete foundations with drilled holes, adding four ground beams to form a unified structure, and reinforcing them with bored piles. Finally, a jacking method is used to correct the tower's on-site deviation. In the Guangdong power grid, most towers still have lightning protection wires installed on top. For these towers, this construction method without power outages is feasible. Compared with the previous option, its advantages are: a) relatively low cost, with an initial contract budget of RMB 370,000 (including land acquisition, road construction, and temporary power supply), and the power supply company does not need preliminary preparation work and can quickly start construction; b) short power outage time, which can be controlled within 2-8 hours, with less impact on the power grid, which is even more important for hub lines like the Siduan Line. 3.1.3 Construction Calculation The abnormal tilting of the transmission line towers is mainly caused by uneven settlement of the tower foundation. Referring to the original design drawings, considering the vertical force, bending moment, and shear force transmitted from the superstructure, plus the weight of the bearing beam and the bending moment transmitted to the bottom of the bearing beam, the vertical ultimate bearing capacity and pull-out force of the single pile, and the bending and shear force of the abutment beam were calculated. Through the study and comparison of the two solutions mentioned above, the Zhaoqing Power Bureau finally adopted a construction plan of reinforcement and correction on the original foundation. 3.2 Construction Methods 3.2.1 Foundation Reinforcement Based on the building's structure and geological deformation, the foundation design utilizes bored piles for support. Each slab is designed with four Φ600 mm bored piles around its perimeter to provide bearing capacity. This approach has the advantages of not damaging the original foundation, avoiding impact loads during construction, minimizing vibration, and having minimal impact on the power transmission towers; it also requires minimal space, allows for flexible operation, and makes foundation reinforcement and correction safe and reliable. Furthermore, it eliminates the need for production or power outages during reinforcement construction. Since the original foundation's four slabs were made of plain concrete, the force could not be directly transferred to the bored piles. Therefore, the design employed the principle of post-tensioning prestressing, drilling horizontal holes in the concrete slabs for reinforcement anchoring, forming four ground beams that connect the piles, columns, and slabs into a unified load-bearing structure. After construction, the foundation forms a "well"-shaped integrated load-bearing structure, significantly improving its resistance to changing loads. 3.2.2 Foundation Correction a) Correction Construction Plan After the overall foundation reinforcement is completed, the horizontal elevation and height difference of the four foundation slabs are determined again based on on-site measurements. The highest horizontal elevation pier IV is used as the reference surface. For the remaining three piers, the higher portion is removed using the jacking and chiseling method according to the height difference. Then, the tower is leveled by using the self-weight of the tower and alternating jacking to support its lowering. b) Correction Construction Site Layout After the piers are treated using the chiseling method, three 200-gauge channel steels are horizontally installed at the main material connecting plate of the tower foot as load-bearing beams for tower foot lifting and adjustment, ensuring balanced stress on the tower foot. One jack is used at each pier (two jacks are used at piers II and III with larger height differences) for pre-jacking, and the anchor bolts are loosened appropriately. c) Correction Construction Process During correction, the height difference is adjusted from largest to smallest. Tower foot III is adjusted first. The two jacks at pier III are lowered by 3 mm each. After tower foot III is fully lowered, the jack throttles are locked. Following this method, lower tower legs II and I by 3 mm respectively. After on-site personnel confirm that there are no abnormalities in both longitudinal and transverse directions, the next round of adjustment will begin. In this way, the three tower legs will be lowered in turn in increments of 3 mm each cycle. The gaps between the lowered tower legs and the foundation will be filled with iron plates until all three tower legs are in place. The tower correction layout is shown in Figure 1. After the correction process is completed, and after on-site measurement confirms that the tower body has been completely straightened, the hollowed-out foundation surface can be re-poured with high-strength concrete, and the entire correction construction will be completed. [img=250,351]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gddl/2002-1/69-1.jpg[/img] 3.3 Construction Safety Measures a) Loosen the fixing end of the lightning protection wire at the top of the tower. In this project, communication optical cables and lightning protection wires are installed on the top of tower No. 012. During the correction, the top of the tower needs to be moved 725 mm in the opposite direction. To ensure absolute safety, the power line was cut off during the correction construction, and the optical cable and lightning protection wire were separated from the tower body. b) Temporary reinforcement. Before drilling and pile drilling, temporary climbing wires were installed on the four sides of the tower. During construction, the stress changes of the climbing wires were closely monitored, and construction was stopped immediately if any abnormality was found. c) Drilling and reinforcement. Horizontal holes were laid out according to the construction drawings. Since the main beam reinforcement passes through the original foundation, the holes must be in a straight line. Horizontal drilling was used, and micro-expansion cement pressure grouting was used for reinforcement anchoring. d) Pile first, then beam. The foundation reinforcement used Φ600 mm cast-in-place piles for replacement. Drilling piles required high speed and strict construction, without interruption, and required one-time completion. After all pile foundation reinforcement was completed, the soil on the slab was cleaned before the main beam reinforcement was carried out. This minimized the possibility of displacement or settlement of the original foundation due to construction. e) Close monitoring. During the tower alignment process, the tower was leveled by loosening the anchor bolts, using a theodolite for alignment, and a level to control the elevation. Before leveling, a horizontal load-bearing beam was installed, and jacks were used to control the tower's descent, ensuring that the tower body did not undergo new deformation due to construction. [b]4 Conclusion[/b] Strengthening and aligning transmission line towers on their original foundations is a relatively novel approach in the Guangdong power system. From its application, its technical and economic effects are quite good: a) It saves money, reducing construction costs by about 50% compared to traditional methods; b) Construction safety is guaranteed; c) It features uninterrupted or minimal power outages; d) The foundation treatment is effective after the alignment is completed. For transmission lines in the Pearl River Delta region, where swampy and silty geology are common and short power outage times are required, the comprehensive benefits of this construction technology should be quite significant.